This invention relates to cold cathodes, which are devices which, without external heating and on application of a relatively small voltage, emit electrons into a vacuum. The invention includes a method of preparation, and also new cold cathodes whose emission characteristics are improved, in some cases by an order of magnitude, over any silicon cathodes described in the literature.
There are two main approaches to forming cold cathodes. One is by the production of negative electron affinity surfaces, and the other by forming material into small pyramids or columns, each with a very sharp point, on the surface of a wafer. This invention is concerned with the latter technique, the provision of sharp tips on a surface.
In order to emit electrons by field emission, the cathode tips must be very sharp, particularly if low operational voltages are required. The electrons are attracted to an anode and a metal gate usually held 0.1 .mu.m to 0.5 .mu.m away is normally used to switch the electron beam on and off. A diagram of a vacuum triode is shown in FIG. 1 and illustrates one possible arrangement of a device. A field emitter is fabricated of metal or semiconductor 10, and includes a cathode tip 12. A metal gate 14 is held around the top of the cathode tip by an insulating layer 16 (of an oxide) and a metal anode 18 is held above the cathode by a further insulating layer 20. When a positive potential difference is applied between the base 10 and the gate 14, an electric field is generated at the tip 12 which allows electrons to tunnel from the cathode material to a vacuum 22. The field at the tip and so the number of electrons emitted are controlled by the gate potential. This basic unit is usually integrated into a very large array, for example as shown in FIG. 2. This comprises a silicon base 24 having a profiled upper surface with silicon pyramids 26. An overlying layer of insulator 28 1 .mu.m thick is itself overlain by a metal grid 30, both gated to reveal the pyramids. The pyramids are shown 10 .mu.m apart, but the packing density of units into the array will depend on the particular application.
The field emission triode shown in the Figures may be used to perform similar functions to a transistor, and there are many applications which have been suggested for vacuum microelectronic devices which may lead to the development of a whole new industry. Possible applications include flat panel displays; superfast computers and memories; a new class of electron sources with large current densities, low extraction voltages, integral focussing and deflection, optical excitation and possibly multiple beams from a single chip; very high frequency amplifiers operating in the GHz range; sub-picosecond electronic devices and high power fast switches; in scientific instrumentation such as electron microscopes and in high radiation environments; for millimeter wave amplification and microwave sources for radar; as pressure sensors; and in electron beam processing of materials and for high gradient accelerators.
The properties which must be successfully developed for the evolution of vacuum microelectronics technology are cold emission, low voltage operation, high current density and small size and compatibility with present-day devices. Low emission noise, long life and uniformity are also required. Developing a fabrication method which gives reproducible cathode geometry and emission, controlling and understanding the physical processes at the emitter surface and practical aspects relevant to real devices, e.g. noise, life time and packing requirements, have all proved to be problems and are taking longer to resolve than expected. This invention focuses on improving the current from and operating voltage of individual cathodes, and also the reproducibility of emission from different individual cathodes; the current density and operating voltage of an array of cathodes should be improved comparably.
Field emitter arrays were first fabricated in 1961. These were of molybdenum and since that time, metals, semiconductors and semiconductors with a metal coating have been investigated for use as the cathode material. Different researchers often use widely differing anode-cathode distances, making it difficult to compare various results in the literature. Currents of 90 .mu.A per tip at an operating voltage of tens of volts have been achieved from solid molybdenum cathodes. The highest current obtained from an n-type silicon is 8 .mu.A at an operating voltage of 750 V. Metal coated silicon tips have produced a maximum emission current of 35 .mu.A, from a tungsten coated tip at an operating voltage of 200 to 330 V.
Metal cathodes can self destruct as they operate at higher currents. Emission uniformity from tip to tip is harder to achieve with metals, due to the stronger field dependence on tip radius and a large metal charge density in the conduction band. Semiconductor arrays can be fabricated using conventional techniques. Silicon is also easier to integrate with present-day devices.
Most geometries which have been examined have been either approximately conical (including pyramidal) or wedges, but rod like geometries have also been investigated. If a conical and wedge emitter have the same base area and the same tip-anode spacings and the same applied voltage, the wedge will generate less current. If the electric field is made the same as that of the conical tip, the field emission current will be considerably larger. Rod-like cathodes have been developed by etching eutectic compositions. These may give greater packing densities but the cathodes are often randomly distributed and would be complicated to integrate with present-day solid state devices.
In many situations the ideal field emitter will produce the highest possible emission current at the lowest possible applied electric field with the smallest possible linear dimensions. FIG. 3 shows various possible field emitter profiles, with a figure of merit f applied to each. A large figure of merit implies a good field emitter, so the best shape shown is the rounded whisker a) and the worst is the wide-angle pyramid d). However, it is also necessary to consider the ultimate limit of field emission current due to electrical breakdown which is determined by the thermal stability of the field emitter, when heat is generated by the electric current. The best shape for this purpose is a wide-angle pyramid and the worst shape a rounded whisker. This is because the temperature gradient of an emitter is largest at the root. Taking account of both factors, an ideal profile for a field emitter is a rounded whisker with a wide base, the Eiffel Tower shape shown in FIG. 4. (C. T. Utsumi, Transactions on Electron Devices, Volume 38, No. 10, October 1991, pages 2276-2283). The radius of curvature of the tip needs to be less than about 50 .ANG., typically in the range 5 to 25 .ANG., the smaller the better.
Porous silicon is a product that has been known since the late 1950s, but has been investigated intensively over the last 15 years on account of its interesting electrical properties including the ability to photoluminesce at room temperature. Porous silicon is formed by anodising silicon in a solvent having some dissolving power for the silicon, typically one based on hydrofluoric acid. The pores typically have diameters of 1 to 100 nm, usually a few tens of nm. The thickness of the resulting sponge structure depends on the anodising time. Control over silicon dopant type, resistivity, current density and HF concentration can be used to control density and other properties of the porous silicon (M. I. J. Beale et al., Applied Physics Letters, Volume 46(1), January 1985, pages 86-88). Following the formation of pores by electrochemical dissolution, chemical dissolution can be used to reduce the density by enlarging the pores until the intervening pillars are separate and form a foam or whiskered structure (L. T. Canham, Applied Physics Letters, Volume 57(10), September 1990, pages 1046-1048).